1. Field of the Invention
The present invention relates to a refrigerator, and particularly relates to a gas cycle engine for a small-sized, low-vibration and long-life refrigerator.
Apparatus such as an infrared image device requires a small-sized refrigerator for the purposes of reduction of thermal noise, improvement of performance, and the like. For such purposes, a Stirling refrigerator using an inverse-Stirling cycle is used widely. The Stirling refrigerator will be described hereinafter as an example, but the present invention is not limited to the Stirling refrigerator.
2. Description of the Related Art
A conventional Stirling refrigerator will be described hereunder with reference to FIGS. 3A and 3B.
FIG. 3A shows the basic structure of the conventional Stirling refrigerator. The Stirling refrigerator has a compressor 51, an expander 52, and a communicating fluid line 53 for connecting the compressor 51 and the expander 52 to each other. The compressor 51 includes a cylinder 56, and a piston 57 which reciprocates within the cylinder to define a compression chamber 58 between the cylinder 56 and the piston 57. The expander 52 includes a displacer 61 provided in another cylinder 59 so as to be able to reciprocate within the cylinder 59. A regenerative heat exchanger 62 is contained in the displacer 61. The regenerative heat exchanger 62 has a gas passage for making a high-temperature orifice 64 communicate with a low-temperature orifice 65. Gas passing through the gas passage is subjected to heat exchange in the regenerative heat exchanger 62. An expansion chamber 66 is defined between the cylinder 59 and the low-temperature end of the displacer 61. Though not shown, the piston 57 is elastically supported by a spring or an equivalent, if necessary and driven by a prime mover with crank mechanism or by a linear motor. The displacer 61 is supported by a spring or an equivalent so as to be elastically movable. The piston 57 and the displacer 61 reciprocate in a state of out-of phase to perform an inverse-Stirling cycle.
The principle of operation of the inverse-Stirling cycle is constituted by the four strokes of isothermal compression, isochoric transfer, isothermal expansion and isochoric transfer of refrigerant gas.
Specifically, it is now assumed that the displacer 61 is initially placed in a neutral position by a spring. When the piston 57 moves to the right in the drawing, refrigerant gas flows into the expander 52 through the fluid passage 53 while isothermally compressed in the compression chamber 58. At this time, the displacer 61 moves to the side of the expansion chamber 66 because of the fluid resistance force of the gas passing through the regenerative heat exchanger 62 from the high-temperature orifice 64 to the low-temperature orifice 65. That is, as the space volume of the expansion chamber 66 decreases, the operating pressure of the whole system reaches its maximum. The process up to this condition is of the isothermal compression.
When the piston 57 has reached the right end, there is no flow of gas. Thus, pressure is balanced between the compression chamber 58 and the expansion chamber 66, and the fluid resistance force does not act. Therefore, the displacer 61 tends to return to the initial neutral position by the spring, and compressed gas moves to the compression chamber 58. Approximately, this process can be deemed as an isochoric transfer which means that compressed gas has moved to the expansion chamber 66 in the condition that there is no change in volume. In this condition, the refrigeration of refrigerant gas does not yet occur.
When the piston 57 is reversed to the leftward as in the initial state, approximately the displacer 61 simultaneously moves to the left because of the fluid resistance force acting on the displacer 61. At this time, the space volume of the expansion chamber 66 increases and the pressure thereof decreases. Thus, expansion occurs. That is, this state can be deemed as an isothermal expansion (in which heat should be absorbed from the outside for achieving an isothermal expansion). Thus, refrigeration occurs.
At the time when the piston 57 has reached the left extreme point, the displacer 61 tends to return to the initial neutral position by the spring because there is no action of fluid resistance force. The gas in the expansion chamber 66 may be transferred isochorically. That is, the refrigerated gas in the expansion chamber 66 returns to the compression chamber 58 via the low-temperature orifice 65, the regenerative heat exchanger 62 and the high-temperature orifice 64, thus completing the four-stroke cycle. In practice, perfect isothermal condition as described above is not always obtained in the respective strokes in the actual operation of the refrigerator.
Because the expansion chamber 66 is used as a source of cold or refrigeration in the aforementioned Stirling refrigerator, an object to be refrigerated is disposed at and thermally connected to the expansion chamber 66. For example, an infrared imaging device is placed at the righthand end of the expander 52. When, for example, the infrared imaging device vibrates by 10 .mu.m, the resulting image will become foggy and blurry. Therefore, low vibration is required for the refrigerator. Further, a long life is required for the refrigerator.
In the structure shown in FIG. 3A, both the piston 57 of the compressor 51 and the displacer 61 of the expander 52 reciprocate and form sources of vibration.
A proposal has been made in which a pair of expanders are arranged in opposition to each other so as to put the object therebetween in order to reduce expander vibration (as disclosed in Japanese Patent Laid-Open Nos. Sho-62-138659 and Hei-2-29556).
Even if such expander vibrations could be prevented, the vibration of the Stirling refrigerator as a whole would not be cleared away as long as the compressor could transmit vibration. As measures to reduce the compressor vibration, a proposal has been made in which a pair pistons are driven by linear motors without crankshaft and are arranged symmetrically to each other (as disclosed in Japanese Patent Laid-Open No. Sho-63-148055).
An example of the structure of the linear motor driven symmetric piston type compressor is shown in FIG. 3B. Two identical pistons 57a and 57b are arranged symmetrically to each other in a cylinder 56 to define a compression chamber 58 therebetween. A yoke 72 having an H-shaped section is arranged around the cylinder 56.
As shown in the sectional view of FIG. 3B, the yoke 72 is constituted by two coaxial cylindrical members and a flange-like member for connecting these cylindrical members with each other. That is, the yoke 72 forms cylindrical spaces both in the left hand side and in the right hand side in the drawing. Further, a gas passage for connecting the compression chamber 58 to the outside is provided in the center portion of the yoke. A pair of ring-shaped permanent magnets 71a and 71b are connected on the internal surface of the outer cylindrical member of the yoke 72. That is, the permanent magnet 71a and the lefthand side portion of the yoke 72 constitute one magnetic circuit, and the permanent magnet 71b and the righthand side portion of the yoke 72 constitute another magnetic circuit.
These magnetic circuits form a pair of magnetic gaps 73a and 73b between the permanent magnets 71a, 71b and the inner cylindrical portion of the yoke 72, respectively. A pair of cylindrical moving coils 74a and 74b are inserted into the pair of magnetic gaps, respectively. An alternating electric current is supplied to the pair of moving coils 74a and 74b through lead wires 75a and 75b. The pair of moving coils 74a and 74b are mechanically connected to the pistons 57a and 57b, which are disposed symmetrically in the cylinder, so that the pistons 57a and 57b are driven by forces acting on the pair of moving coils 74a and 74b, respectively.
The pistons 57a and 57b are linearly driven by linear motors constituted by the moving coils 74a and 74b, the permanent magnets 71a and 71b and the yoke 72. Accordingly, each of the pistons does not receive a force acting in a direction perpendicular to the direction of the movement of the piston, compared with the case of crank driving. As a result, not only the life span of bearings and sealings for the pistons is prolonged but vibration is reduced. Further, because the pistons 57a and 57b move in opposition direction, forces from the two pistons cancel each other to reduce vibration.
Heretofore, the support pistons and displacers has been given by coil springs or equivalents. In recent years, supporting of a moving member by a leaf spring or springs has been developed. FIG. 4 shows an example of the Stirling refrigerator in which moving members are supported by parallel leaf springs.
An opposite piston type compressor 51 shown in the lower half of the drawing is connected, through a communication pipe 53, to an expander 52 shown in the upper half of the drawing. In the compressor 51, pistons 57a and 57b are disposed in opposition to each other and driven by shafts 81a and 81b, respectively. Moving coils 74a and 74b are connected to the shafts 81a and 81b and inserted into gaps of magnetic circuits constituted by permanent magnets 71a and 71b and yokes 72a and 72b, respectively.
The shafts 81a and 81b are supported by an outer casing through two pairs of leaf springs (82a, 83a) and (82b, 83b), each being made of disk-shaped thin elastic metal diaphragm. These leaf springs 82 and 83 elastically allow displacements of the shafts 81a and 81b in the axial direction. An electric current is supplied to the moving coils 74a and 74b through lead wires 75a and 75b, respectively.
In this structure, the expander 52 connected through the communication pipe 53 to the compressor 51 also has a driving mechanism constituted by a linear motor. That is, a shaft 84 for supporting a displacer 61 having a regenerative heat exchanger 62 is supported by the outer casing through diaphragm-shaped leaf springs 85 and 86 and mechanically connected to a moving coil 88.
The moving coil 88 is disposed in a magnetic gap of a magnetic circuit constituted by a permanent magnet 89 and a yoke 90, so that the moving coil 88 produces a force in the axial direction when a current passes through the moving coil 88. The current is supplied to the moving coil 88 through lead wires 91. By using such disk-shaped leaf springs, it is possible to support the pistons and displacers on the axis of movement with high accuracy and it is possible to inhibit the displacement in the direction perpendicular to the axis of movement. Accordingly, the abrasion or wearing-off of piston sealings or equivalents can be reduced greatly.
Further, when the aforementioned centering guide mechanism is employed, compression pistons can be supported in a non-contact manner. Further, clearance sealing for providing a narrow gap between a piston and a cylinder to eliminate the physical sealings and to reduce the quantity of gas leakage as much as possible can be used for prevention of leakage.